Line of sight (LOS) microwave MIMO communications typically involve two or more antennas separated by several meters. Various attempts to design LOS MIMO systems that can work with non-coherent frequency sources have not been successful. The alternative is LOS MIMO systems based on coherent frequency sources. However, the distribution of coherent frequency sources to multiple outdoor units for MIMO transmission or reception remains a significant hurdle for microwave MIMO systems.
In systems where a single frequency source is used with no special distribution mechanism, only a very small separation between antennas is possible resulting in low MIMO capacity.
To achieve better MIMO capacity, a larger separation between antennas of the outdoor units is required. In such systems, starting with a single frequency source, it is difficult to get the required coherence between the signals at the two antennas. Distribution of a single signal to multiple outdoor units over waveguides is costly and involves difficult installation procedures. Distribution over coaxial cable is costly and lossy.